1. Field of the Invention
The present invention relates to means for improving laser performance through reduction of amplified spontaneous emission (ASE), and more particularly, it relates to the use of a flanged and tapered laser rod to limit the maximum ray paths that can be trapped within the laser rod.
2. Description of Related Art
As early as 1965, reports in the literature discuss the impact of trapped light in polished barrel laser rods (J. Linn and J. Free, “Effect of Trapped Light on the Output of a Ruby laser,” Applied Optics, 4, p 1099, 1965). The established picture of trapped laser light 10 in a polished barrel laser rod 12 is shown in FIG. 1. Nearly all laser rods employed in laser systems today incorporate fine grind on the side surfaces of the laser rod to disrupt and scatter the barrel modes. The new approach described herein functions with an optical polish on the side surfaces of the laser rod and instead sweeps the barrel mode light to the one of the ends of the laser rod, where it is ejected.
Trapped by total-internal-reflection (TIR), light can swirl around the inside of the laser rod and travel over long path lengths before reaching the end of the rod. The result of these long trapped ASE paths is that the gain present in the outer portion of the laser rod can be effectively depleted. If nr is the refractive index of the laser rod and ns is the refractive index of the material surrounding the laser rod, then it can be shown that the annular portion of the rod swept out by these swirling rays is given by (ns/nr) rrod<r<rrod. This means that if the gain depletion is severe, then only that portion of the rod lying inside a circular area with radius (ns/nr)rrod is useful for extracting laser energy from the rod. FIG. 2 illustrates the useful laser energy extraction area 20 and the unusable area 22.
As an example, consider a YAG rod surrounded by water for cooling, where nr=1.82 and ns=1.33. In this case, if the swirling rays cause severe gain depletion, then only that portion of the rod contained within the central 53% (=100(1.33/1.82)2) of the rod's cross sectional area would be useful for contributing to laser output power.
One technique that has recently been developed and demonstrated, the flanged end-capped laser rod See U.S. Pat. No. 5,936,984, “Laser Rods With Undoped, Flanged End-Caps,” by Helmuth Meissner et al., incorporated herein by reference), has proven useful in limiting the maximum length of the ray path that can be trapped in the laser rod. In the case of the straight barrel polished laser rod shown in FIG. 1 above, if the swirling rays of light wander to the end of the laser rod and then strike the end face of the rod at a high angle, they will be completely trapped by total internal reflection at that end face. In such a situation, the rays will be simply turned around and sent swirling down the rod in the other direction until they hit the output face at that end. This process is particularly bad from the viewpoint of depleting the stored laser energy in the rod, as these trapped rays now have an infinitely long trapped path length.